Sorel cement is a term used to refer to various cured compositions having as basic ingredients a combination of magnesia (MgO) and magnesium chloride (MgCl.sub.2). This basic Sorel system when cured is a magnesium oxychloride hydrate.
Sorel cement was discovered almost 100 years ago. It gets harder, and sets faster than Portland cement, but its widespread use has been greatly limited because of its inherent poor water resistance. The cured Sorel cement as it is known today is also somewhat soluble in water with the result that exposure to water virtually eliminates the adhesion between the crystals.
It is obvious from repeated statements in the literature that had it not been for the water sensitivity of Sorel cement products, their use would have been much more general and widespread. It is exactly because of this drawback of these cement products that there remains a large potential for these materials if the water sensitivity problem could be solved. The superior hardening rate, greater strength and excellent fire retardant properties of Sorel cement could then be taken advantage of in a host of building materials where its use is presently not considered.
Various attempts have been made to overcome this difficulty by the addition of materials which have the property of forming insoluble magnesium salts, such as phosphates and aluminates. Materials such as waxes, oils and metal soaps have also been added to improve water resistance (See U.S. Pat. Nos. 2,703,762 and 3,753,750, for example). The results have been only partially successful and in fact usually with the further disadvantage that the hardening rate is greatly slowed.
Phosphoric acid and phosphates have been added to Sorel cements for various reasons, such as to improve wet strength, to control viscosity, to reduce contraction upon curing, etc. The prior art presently known to the applicant involving the addition of phosphoric acid or phosphates to Sorel cements are U.S. Pat. Nos. 4,141,744; 3,320,077; 2,702,753; 2,543,959; 2,351,641; 1,853,522; 1,019,083, and a Supplementary Paper Section III, THE VI INTERNATIONAL CONGRESS ON THE CHEMISTRY OF CEMENT, Moscow, September, 1974.
Some of the above problems have been solved as disclosed in U.S. Pat. No. 4,209,339 dated June 24, 1980, by the present applicant. That patent concerns the use of ethyl silicate additive to improve water resistance and to the use of a premix of water, magnesium chloride, and a small amount of magnesium oxide with which the remainder of the Sorel cement ingredients are mixed.